A simplified diagrammatic representation of a disk drive, generally designated as 10, is illustrated in FIG. 1. The disk drive 10 includes a data storage disk 12 that is rotated by a spindle motor 14. The spindle motor 14 is mounted to a base plate 16. An actuator arm assembly 18 is also mounted to the base plate 16.
The actuator arm assembly 18 includes a transducer 20 (or head) mounted to a flexure arm 22 which is attached to an actuator arm 24 that can rotate about a pivot bearing assembly 26. The actuator arm assembly 18 also includes a voice coil motor (VCM) 28 which moves the transducer 20 relative to the disk 12. The spindle motor 14, and actuator arm assembly 18 are coupled to a number of electronic circuits 30 mounted to a printed circuit board 32. The electronic circuits 30 can include, for example, a digital signal processor (DSP), a microprocessor-based controller and a random access memory (RAM) device.
Although a single disk 12 is illustrated in FIG. 1, the disk drive 10 may instead include a plurality of disks. For example, FIG. 2 illustrates a disk stack 15 that includes a plurality of disks 12, each of which may have a pair of data storage surfaces 36. The disks 12 are mounted on a cylindrical shaft and are designed to rotate about axis 38. The spindle motor 14 as mentioned above, rotates the disk stack 15.
Referring now to the illustration of FIGS. 1-3, the actuator arm assembly 18 includes a plurality of the transducers 20, each of which correspond to one of the disk surfaces 36. Each transducer 20 is mounted to a corresponding flexure arm 22 which is attached to a corresponding portion of the actuator arm 24 that can rotate about the pivot bearing assembly 26. The VCM 28 operates to move the actuator arm 24, and thus moves the transducers 20 relative to their respective disk surfaces 36. The transducers 20 are configured to fly adjacent to the disk surfaces 36 on air bearings.
FIG. 4 further illustrates one of the disks 12. Data is stored on the disk 12 within a number of concentric tracks 40 (or cylinders). Each track is divided into a plurality of radially extending sectors 42 on the disk 12. Each sector 42 is further divided into a servo sector 44 and a data sector 46. The servo sectors 44 of the disk 34 are used to, among other things, accurately position the transducer 20 so that data can be properly written onto and read from the disk 12. The data sectors 46 are where non-servo related data (i.e., user data) is stored and retrieved. Such data, upon proper conditions, may be overwritten. Unlike information in the data sectors 46, the servo sectors 44 should not be overwritten or erased during normal operation of the disk drive 10.
To accurately write data to and read data from the data sectors 46 of the disk 12, it is desirable to maintain the transducer 20 at a relatively fixed position with respect to a centerline of a designated track 40 during writing and reading operations (called a track following operation). To assist in controlling the position of the transducer 20 relative to the tracks 40, the servo sectors 44 contain, among other things, servo information in the form of servo burst patterns that include groups of servo bursts, as is well-known in the art.
A servo burst pattern 50 that includes first, second, third and fourth servo bursts A, B, C and D, respectively, is shown in FIG. 5. The servo burst pattern 50 can be located at, for example, radial locations n−1 to n+4, and are drawn in a straight, rather than arcuate, fashion for ease of depiction. The servo bursts A, B, C, D are accurately positioned relative to each other. Although not illustrated, the servo sector 44 may also include a write/read (W/R) recovery field, an automatic gain control (AGC) field, a synchronization field, a sector number field, and/or a cylinder number field.
During the manufacturing process of the disk drive 10, a servo-track writer (“STW”) (not shown) can is used to write the servo bursts A, B, C, D onto each of the servo sectors 44 of the disk 34. In FIG. 5, the distance (d) between each pair of horizontal grid lines represents ½ of the servo track pitch. Additionally, as depicted in FIG. 5, the transducer 20 has a width approximately equal to one-half of the servo track width. The transducer 20 is shown to be misaligned from the track centerline 48 of track n−1 to more clearly illustrate an example of its width.
As the transducer 20 is positioned over a track, it reads the servo information contained in sequential ones of the servo sectors 44 of the track as a read signal. The servo information is used to generate position error signals as a function of the misalignment between the transducer 20 and the track centerline 48. The position error signal is provided to a servo controller that performs calculations and outputs a servo compensation signal which controls the voice-coil motor 28 to position the transducer 20 relative to the track centerline 48.
When the transducer 20 is positioned exactly over the centerline 48 of track n, approximately one-quarter of the A burst will be read followed by one-quarter of the B burst, and their amplitudes will be equal in the read signal. As the transducer 20 moves off-track (i.e., off of the track centerline), the amplitude of one burst will increase while the amplitude of the other burst will decrease, depending on the direction of misalignment. Accordingly, the radial position of the transducer 20 relative to the tracks can be determined based the servo information in the read signal from the servo bursts A, B, C, D.
While reading and writing data on the disk 12, the transducer 20 rides above the disk surface on a cushion of air (known as an air bearing) that is created by the movement of the disk 12 under the transducer 20. The distance of the transducer 20 from the disk 12 while riding on the air bearing is referred to as the “flying height” of the transducer 20. Transducer “sliders” are generally used that have the requisite aerodynamic qualities to produce the “lift” needed to hold the transducer 20 away from the disk 12. In general, the performance and/or operation of the disk drive 10 can become degraded if a proper flying height of the transducer 20 is not maintained. For example, the read signal can become unreliable if the actual flying height of the transducer 20 is considerably higher than a nominal flying height, and undesirable contact of the transducer 20 and disk 12 can occur if the flying height is too low. Fly height can vary from one transducer 20 relative to disk surfaces within the disk stack 15, and can vary based on radial position of the transducer 20, air density (ambient temperature) and transducer 20 temperature.